Bool Tool Add-on missing option
The Exact solver option will be introduced with the release of Blender 2.91 as documented by the release notes. The design task for the boolean redesign was T67744 and the commit that introduced this to the master branch rB9e09b5c418c0a436e3c84ccf38c065527988b0a0.
The release of Blender 2.91 is planned for the end of November 2020. You can download the daily builds of the beta version from builder.blender.org.